The present invention relates to a solid oxide fuel cell stack configuration incorporating floating cells.
High temperature fuel cells like solid oxide fuel cells comprise an electrolyte sandwiched between a cathode and an anode. Oxygen reacts with electrons at the cathode to form oxygen ions, which are conducted through the ion-conducting ceramic electrolyte to the anode according to the reaction:½O2+2e→O2−  (1)
At the anode, oxygen ions combine with hydrogen and carbon monoxide to form water and carbon dioxide thereby liberating electrons according to the exothermic reactions:H2+O2−→H2O+2e  (2)CO+O2−→CO2+2e  (3)
The fuel cells are stacked and interleaved with interconnect plates which distribute gases to the electrode/electrolyte interfaces and which also act as current collectors.
Planar solid oxide fuel cells are believed to potentially offer lower cost and higher power densities per unit volume compared to tubular designs. However, planar SOFC designs face many challenges in materials development, processing, and system integration that must be overcome. Sealing a planar SOFC stack is a particularly difficult problem. The seals must provide sufficiently low leak rates to prevent fuel combustion in the air stream, which can lead to structural failure of the stack. The seals must also be stable over a long service life and not cause degradation or alteration of materials which contact the seals. Finally, the seals must be able to survive thermal cycling of the stack during routine operations.
Composite flexible seals have been developed which are better able to handle thermal cycling, but these seals must be compressed to provide adequate sealing performance. Conventionally in planar SOFC stack design, the sealing layer comprise seals together with a contact media between a fuel cell electrode and an interconnect or current collector plate. Balanced compressive loading on both seals and contact media of the fuel cell is then required. If the compressive loading is greater on the seals than the contact media, there may be insufficient electrical contact between the electrode, contact media and the interconnect or current collector plate to achieve good electron migration. On the other hand, if the compressive loading is greater on the contact media than the seals, shear forces may crack the fuel cell or causing leakage due to insufficient sealing.
Furthermore, even if a fuel cell stack is assembled in a “balanced manner”, operation of the stack and thermal cycling may quickly unbalance the compression force in the stack, causing loss of electric contact, seal leakage or fuel cell cracking, or other problems.
Therefore, there is a need in the art for fuel cell stack configuration, which mitigates the difficulties in the prior art.